1. Field of the Invention
This invention pertains to the acquisition of electroseismic data and more specifically to the acquisition of such data by the development and detection of electromagnetic waves.
2. Description of the Prior Art
Most seismic prospecting is accomplished by the development of acoustical waves from one or more seismic sources located at or near the earth's surface. These acoustic waves are known to be reflected by interfaces or discontinuities in the subterranean formations so as to be returned to the earth's surface to be detected by one or more appropriately positioned seismic or acoustical detectors, normally geophones. It is known that some of the reflected waves are so-called shear waves (s-waves) and other of the reflected waves are so-called compressional waves (p-waves), differing from each other in their respective angles of reflection and the acoustical vibrational directions of the particles in the layered formations through which the waves pass. Significantly, both wave types travel basically at the speed of sound and are attenuated by the formations through which they pass in the same manner as any other sound waves in the same frequency range, which are in the low frequency range for relatively deep prospecting since it is well known that the higher frequencies are greatly attenuated by the formation media.
Attempts have been made over the years to employ a scheme for seismic prospecting that used other phenomena than acoustical detection with geophones as just described. For example, as early as 1936, a procedure was patented by L.W. Blau, et al, U.S. Pat. No. 2,054,067, that utilized resistance modulation near the earth's surface resulting from a seismic blast to cause a detectable electromagnetic response. Basically, the formations near the surface are stratified into layers having different densities. A seismic blast would cause each layer of its particular constituent structure to modulate in accordance with its density i.e., its porosity makeup. Thus, the detected electromagnetic field at a particular location, being proportional to the resistance modulation within the field, is an indication of how thick the respective layers of different densities are at that particular location. The technique described in the '067 patent was not greatly commercialized, if at all, probably because it was useful only at shallow depths in the vicinity of the surface and not useful to any significant depth where the majority of interest is in oil and gas prospecting. This early work by Blau et al was directed at the measurement of the seismically induced resistance modulation of the formation by passing an electrical current through (or applying a voltage to) the earth and then measuring the modulation of that current (or the modulation of the resulting voltage). As will be evident, this method is distinctively different from the procedure described herein basically because no current is applied to the formation or earth surface.
Another technique that has been employed with respect to the detection of certain mineral deposits utilizes a continuous wave seismic source that induces a voltage in the deposit due to the piezoelectric effect. In such a case, the seismic wave distorts a piezoelectric formation like quartz, which then is polarized and emits an electromagnetic wave. No fluids are involved. Such techniques utilize relatively high frequencies and, therefore, are limited to a short penetration depth and, further, is useful in detecting only those limited kinds of deposits that exhibit a piezoelectric effect characteristic.
One of the most interesting attempts to develop an alternate technique to the standard seismic acoustic technique is described in U.S. Pat. No. 2,354,659, W.0. Bazhaw, et al, issued Aug. 1, 1944. In accordance with this procedure, a seismic blast downward would encounter a layer of fluid in the subterranean formation located underneath a gas layer and cause the fluid to rapidly rise upward into the presumably porous gas layer. As the relatively slow settling of the fluid (oil and water) occurs, this slow downward fluid movement induces a current change in the path between two electrodes embedded in the earth's surface and connected to a suitable electronic amplifier and recorder. If no liquid is present, then there is no current change. If there is a liquid present, then there is a change depending on the respective parameters of the fluid and formation. Such change can be metered as a dc voltage for an appreciable period of time after the blast until equilibrium is reached. Although different from the Blau et al procedure, the technique involves a procedure useful at only very shallow depths and therefore has little, if any, practical commercial application. This is because (1) the dc voltage will not propagate as an electromagnetic wave and, therefore, is only useful at shallow depths and (2) very low frequencies, essentially dc, have very long wave lengths, which means there is very poor depth resolution. It will be seen that by contrast, the procedure described herein retains the frequency character of a seismic wave.
The basic physical process required for electroseismic prospecting or ESP in accordance with applicant's invention is that seismic energy can be converted to electromagnetic energy of significant value. Although there are several possible theoretical conversion mechanisms that might cause the observed happening, such as resistance modulation, discussed above, spontaneous potentials and electrocapillarity, the mechanism that best explains the observations of applicant and is useful in utilizing the procedure herein described, is referred to as "streaming potential". This mode of conversion of seismic to electromagnetic energy appears to be the theory that is most effective in analyzing what occurs with fluid movement present in a porous lithological formation and is most pronounced when there is the presence of at least two immiscible fluids, such as oil and water or gas and water. The phenomenon also exists in the presence of a lithological structure of high permeability where there is pore fluid in the structure. Basically in accordance with this theory, there is a molecular chemical-bond attraction between the fluid and the porous surface of the solid formation, which bond is distorted or broken with the rapid movement of the fluid upon contact by an acoustical wavefront, thereby inducing in a dipole manner an electromagnetic response. M.A. Biot described the fluid movement accompanying a seismic pressure gradient in papers published by the Journal of the Acoustical Society of America in 1956 and 1962, at page 168 of volume 28 and page 1254 of volume 34, respectively. Others, such as J.0. Bockris and A.K.N. Reddy have experimented with the streaming potential and reported circa 1973 on their findings, but heretofore, the effect has not been utilized in electroseismic prospecting such as set forth herein.
Therefore, it is a feature of the present invention to utilize the "streaming potential" effect in electroseismic prospecting for inducing a detectable electromagnetic field capable of directly revealing the presence of two immiscible fluids, such as an oil and water or gas and water or the presence of a fluid in the pore space of a high-permeability formation.
As noted, this invention pertains to the acquisition of electroseismic data and is referred to sometimes herein as electroseismic prospecting or ESP. Electroseismic prospecting is distinguishable from the operation of an electromagnetic geophone, which senses the presence of a reflected seismic or acoustical wave at the earth's surface. Although electromagnetic geophones were first investigated prior to 1950, their operation did not lead to electroseismic prospecting.
There is an essential distinction that can be made between ESP data and seismic data. Seismic data only reveal structural information related to the elastic contrast between two different lithological regions. No information is revealed about what kind of rock is present or what is in the pore space of the regions under investigation. On the other hand, ESP only works where there is mobile, conducting water in the pore space of the formation under investigation or where there is a mixture of water and hydrocarbon. Therefore, it is clear that ESP is not a special case of seismology, but is fundamentally different. The fact that ESP is sensitive to the type of pore fluid is the source of its usefulness.
Therefore, it is yet another feature of the present invention to detect in an improved manner the presence of mobile, conducting water in the pore space of a lithological formation under investigation or the presence of a mixture of water and hydrocarbon.